The present invention is concerned with a glass substrate for a magnetic disk, and were especially, it relates to a magnetic disk with a surface which has no chemical strengthening layer.
Moreover, the present invention relates to a magnetic disk whose surface roughness in an information recording portion thereof is sufficiently small, and a glass substrate for use in magnetic disk and magnetic disk that are suitable for a high density recording with high reliability. The present invention also relates to the process of manufacturing a glass substrate and a magnetic disk device.
Texturing of a glass substrate of a magnetic disk by using a laser has been attempted recently. While this method involves a technique used on metal substrates, such as Ni-P, this technique also maybe applied to glass. As described in Patent pre-publication 7-182655 and Patent prepublication 9-138942, laser texturing techniques using an ultraviolet light laser of 266 nm or a carbon dioxide laser that has a long wavelength of 10.6 xcexcm have been used for ordinary glass having a transparency to light in the range from infrared to near ultraviolet.
In the above-described glass system, the substrate is reinforced by a chemical strengthening layer, and it is also known that texture formation maybe easily performed by compression of the substrate surface. (Ref. A. C. Tam et. al., IEEE(1997)).
As a recording medium for a note book type personal computer, a 2.5xe2x80x3 glass substrate with a high surface smoothness is installed in a magnetic disk device. The glass substrates implemented at present are chemically strengthened glass substrates and crystallized glass substrates. However, lowering of the traveling height of the magnetic head is necessary to increase the recording capacity or recording density per unit area of the magnetic disk. Therefore, a magnetic disk with a smoother recording surface is being developed.
When the magnetic disk is rotating, the magnetic head floats on the recording surface of the disk, and, when the disk stops, the head lands on and contacts the disk. Thus, sticking may occur when the face where the head lands is too smooth. Therefore, the landing point of the heads on the disk surface must have a proper roughness. For the reason, a texture processing that makes all recording surfaces of the magnetic disk have a proper roughness has been adopted. However, in view of the requirement to provide a small traveling height, the texture should not be formed on the entire recording surface, but should be formed only in a zone (CSS zone; Contact Start Stop Zone) that is formed where the head lands (zone texturing).
A method of etching, sputtering or sol-gel coating only a CSS zone after masking the recording surface has been attempted as a method of effecting zone texturing. However, with these methods, the masking is not sufficient in some cases, and so the recording surfaces become rough to some extent, which reduces the available recording area. To solve this problem, the texture is formed by using a laser. There is a method of providing a laser texture, which is described in Japanese patent Laid-open print No. 7-182655. However, with such method, it was difficult to form the texture with a stabilized configuration and width, because the method uses a carbon dioxide gas laser and the output stability of a carbon dioxide gas laser is not satisfactory. Since the wavelength of the carbon dioxide gas laser is long (10.6 xcexcm), the spot diameter becomes large. Therefore, it is difficult to form a small sized texture.
Sufficiently small bumps that constitute a texture are formed by using ultraviolet light in the method disclosed in Japanese patent Laid-open print No. 9-138942. In this specification, the term bump is used to describe each of a plurality of small projections that constitute the texture of the disk. Because the depth of focus is shallow in this method, focusing on the substrate surface is difficult, so that control of the bump height with a high degree of accuracy is difficult. Because ultraviolet light was absorbed in the ordinary lens system, the loss was large. This produced a difficulty and danger to operators as well, because ultraviolet light cannot be directly observed with the naked eye.
The mechanical strength of the chemically reinforced substrate is high and the formation of the laser texture is easy to achieve in this substrate, but there is a fear that alkali ions with a large ion diameter that are introduced into the glass substrate by the chemical strengthening will move to the substrate surface. As a result, the peeling of a magnetic film from the substrate or sticking may arise. A crystallized glass substrate is conceivable as an example of other strengthened substrates. In case a crystallized glass substrate is used, however, the smoothness of the recording surface is not sufficient, and so it is difficult to obtain a high recording density.
In view of the above, an object of the present invention is to provide a glass substrate with a high chemical stability and to obtain a glass substrate for a magnetic disk for forming a stable laser texture which is suitable for use as a high recording density magnetic disk as well.
Another object of the present invention is to obtain a glass substrate for a magnetic disk having a high mechanical strength with good reliability and which has no chemically reinforced layer.
A further object of the present invention is to provide a magnetic disk having high reliability and a high recording density and a magnetic disk device using the same.
The invention employs the following measures to achieve the above objects. A glass substrate for the magnetic disk of the present invention has an information recording surface to record information on at least a part of the surface thereof, but it does not have a chemical strengthening layer on the surface. The surface roughness Ra of the information recording surface is 2.0 nm or less, and there exists a wavelength zone in which the transmittance of light becomes 10% or less in the wavelength range of from 300 nm to 2000 nm. In other words, the glass substrate should have a transmittance of 10% or less to the laser light being used.
At least one transition metal element, which is selected from the group consisting of titanium, vanadium, chromium, manganese, cobalt, nickel, copper, gold and silver, is contained in the glass substrate. The transition metals are used for absorbing laser light and reducing the transmittance of the glass composition to laser light. When the transition metal element is cobalt, 1 to 30% by weight of cobalt is preferably contained on the basis of the conversion as CoO. In this case, using CoO, the wavelength zone for effecting laser texturing is in the range of from 450 to 700 nm.
The composition of the glass substrate for the magnetic disk of the present invention should contain a rare earth metal, which increases the mechanical strength of the glass substrate. An example of a preferable glass composition is: SiO2: 50 to 80 weight %, B2O3: O to 15 weight %, R2O: O to 20 weight % (R=alkali metal element), Ln2O3: O to 10 weight % (Ln=rare earth element), Al2O3: 0.5 to 15 weight CoO: 1 to 30 weight % on the basis of oxide conversion.
The magnetic disk of the present invention comprises a circular or disk-shaped glass substrate and an information recording film formed directly or through another layer on the surface of the substrate. The substrate surface has a non-information recording area and an information recording area that are formed in concentric relation to the periphery of the substrate. The surface roughness Ra of the information recording surface is 2.0 nm or less, and the maximum surface roughness Rmax is 5 nm or less. The laser texture is formed in the non-information recording area. The structure of the texture should be regularly formed. The mean height of the bumps that constitute this texture should preferably be 10 nm to 25 nm.
A chemical strengthening layer does not essentially exist in the surface part of the glass substrate. The substrate has a transmittance of 10% or less to monochromic light or laser light, such as light having a wavelength of 300 to 2000 nm.
The manufacturing process of the magnetic disk of the present invention comprises the following steps. A substrate, which has no chemical strengthening layer in the surface thereof and has a surface roughness Ra of 2.0 nm or less, is used. The texture is formed in the non-information recording area of the surface of the glass substrate by irradiating it with a laser beam having a wavelength of 300 to 2000 nm so as to form bumps having a mean height of 10 nm to 25 nm. When the glass substrate contains cobalt, the preferable wavelength to be used is 450 to 700 nm. The magnetic disk device of the present invention comprises a magnetic head, a magnetic disk, a spindle motor to rotate the magnetic disk, and a motor for changing the position of the magnetic head in a direction parallel with the surface of the magnetic disk.
The magnetic disk has a glass substrate and a magnetic film formed directly or through another layer on it. The disk surface has a non-information recording area and an information recording area. During rotation of the magnetic disk, the traveling height of the magnetic head on the information recording surface is 30 nm or less, the surface roughness Ra of the information recording surface is 2.0 nm or less, and Rmax is 5 nm or less.
A chemical strengthening layer does not exist on the surface part of the glass substrate. The substrate has a transmittance of light of 10% or less in the wavelength range of 300 to 2000 nm.
According to the present invention, because there is no chemical strengthening layer on the surface, a glass substrate for a magnetic disk that is excellent in chemical stability can be obtained. Moreover, a glass substrate for a magnetic disk on which a laser texture can be formed stably in a predetermined noninformation recording area is obtained. Therefore, a glass substrate for a magnetic disk with a high recording density is obtained. When rare earth elements are added to the substrate, a glass substrate having a high mechanical strength and a high reliability are obtained even though it does not have a chemical strengthening layer. Thus, a magnetic disk and a magnetic disk device with a high recording density and a high reliability,can be obtained.